The vectors form a linearly dependent set in ℜ3 if the determinant of the matrix formed by the three vectors is zero.

Evaluate the determinant:
|x,k,k|
|k,x,k|
|k,k,x|
in terms of x and k, where k can eventually substituted for -1/9.
Solve the resulting cubic equation for x in terms of k. It actually factors nicely into (x+?)(x-??)².